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Cosmological "constant" in a universe born in the metastable false vacuum state

The cosmological constant $Λ$ is a measure of the energy density of the vacuum. Therefore properties of the energy of the system in the metastable vacuum state reflect properties of $Λ= Λ(t)$. We analyze properties of the energy, $E(t)$, of a general quantum system in the metastable state in various phases of the decay process: In the exponential phase, in the transition phase between the exponential decay and the later phase, where decay law as a function of time $t$ is in the form of powers of $1/t$, and also in this last phase. We found that this energy having an approximate value resulting from the Weisskopf--Wigner theory in the exponential decay phase is reduced very fast in the transition phase to its asymptotic value $E(t) \simeq E_{min} + α_{2}/t^{2}+\ldots$ in the late last phase of the decay process. (Here $E_{min}$ is the minimal energy of the system). This quantum mechanism reduces the energy of the system in the unstable state by a dozen or even several dozen orders or more. We show that if to assume that a universe was born in metastable false vacuum state then according to this quantum mechanism the cosmological constant $Λ$ can have a very great value resulting from the quantum field theory calculations in the early universe in the inflationary era, $Λ\simeq Λ_{qft}$, and then it can later be quickly reduced to the very, very small values.